Bluegrass blends with green grass and those shady Valley oak trees as the musical palette expands a bit more during this summer's free concert series at Stockton's Oak Park.

Snap Jackson & the Knock on Wood Players, a Stockton band that's been named California's best bluegrass band, opens the series of 12 Wednesday park concerts on June 5.

"I'm excited," said Jackson, who fondly remembered attending the picnic concerts with his family while growing up. "I really don't know what to expect. I hope people come out to hear us. We're definitely excited to share what we do with local people."

In recent years - as threatening financial difficulties have been resolved and the 61-year-old series has become more stable and secure - its directors have attempted to broaden the musical scope.

As far as anyone can recall, the Knock on Wood Players are the first bluegrass group - though a slightly hybrid one stylistically - to perform since the series began at Oak Park in central Stockton.

"It's great," said Brian Clark, the quartet's upright bass player who's familiar with playing in Victory Park. "We're very honored we got the call and were offered the chance to open. We're very happy. It's a perfect fit: Outside in the park. Good weather. Really, it's the perfect setting for roots and Americana music."

Clark, 37, a graduate of Stockton's Bear Creek High School and University of the Pacific, also plays bass in the Valley Concert Band, a 29-member ensemble conducted by Chris Anderson that's the rock of the series, playing its band-music standards on June 12, July 3 and Aug. 7.

The 12-concert series, which typically attracts in excess of 12,000 people, also includes the classic rock of Summit (July 31), the rhythm and blues of RBX (June 26), the return of country-rockabilly music with the Nick Isaak Band (Aug. 14) and the showmanship of Waterloo, a Stockton-based ABBA tribute band led by drummer John Wells that concludes the series on Aug. 21.

Wells, who also drums in Al Mannon's Swingaires big band, will miss that group's July 24 Victory Park show while he attends a musicians-union convention in Las Vegas.

Wells, president of the Stockton Musicians Association Local 189, said he and members of the co-sponsoring Stockton Concerts in Victory Park Inc. - a nonprofit group that raises funds for the series and chooses the groups via a six-member committee - hopes to develop even more musical diversity.

"I like bluegrass," said Wells, 48, a Stagg High graduate. "I like and know the players very well. I think they're fantastic. Everyone has their own cup of tea.

"There are pros and cons for everything. I'm better than OK with that. That's good. We've got a good representation of lots of genres. But, you know, there are a lot of other talented groups. It's not so much a union-sponsored thing any more. We've got lot of bands who could 'quote-unquote' audition."

Mannon's well aware of that.

"We get calls from different performance groups who are anxious to play out there," said Mannon, a board member for the concert association and union. "It's kind of across-the board. There are plenty of qualified groups. That's been our project since way back in history."

Jackson and Clark's group certainly is "qualified." Formed four years ago, the Knock on Wood players - Jackson, Clark, fiddler Shane Kalbach and guitarist Eric Antrim - perform "at a slew" of California events this summer and at bluegrass festivals in Brunswick, Me., and Cape Cod, Mass.

"It's been a long time coming," said Jackson, 34, a Lincoln High School graduate and multi-instrumentalist whose group was named band of the year for 2012 by the California Bluegrass Association in January. "We're really honored to be that."

Again this year the concert "show-mobile" stage and portable toilets are being provided by the city. Donations cover all other expenses.

For the first time in 2012, no money was available from a national free-music fund. So, Wells "passed the hat" - actually a box - during shows, a method that had been prohibited by the national organization.

"It was phenomenal," Mannon said. "Very successful. We generated almost as much as we would have gotten from the national fund. We're in pretty good shape for this year, but we're getting a little lax. We might have trouble next year."

That's one thing the concerts series helps relieve.

"It's awesome," said Clark, who's trained in classical and jazz. "It's free music in Stockton. Out in the park with people bringing their families and throwing out blankets and bringing chairs. Amid all the ugliness, this is something special. I've never seen a fight out there. People are enjoying something with their families. It's imperative that we have these opportunities in Stockton."